Study of the NaBH4–NaBr system and the behaviour of its low temperature phase transition.

A mechano-chemical method was used to synthesize solid solution Na(BH 4 ) 1-x Br x with 0 ≤ x ≤ 1. Samples with compositions of x ≤ 0.333 were annealed, in order to form a single phase material. Bromide substitution leads to smaller unit cell size and lower temperature and enthalpy of the order-disorder phase transition of NaBH 4 . There is a linear relation between the amount of substitution, the temperature, the enthalpy and the kinetics of the phase transition. This linear relation between enthalpy and amount of substitution can be expressed by the function ΔH = −6.268x + 1.206 where x is the amount of substitution and ΔH is the enthalpy. [ABSTRACT FROM AUTHOR]